Quantitative analysis of elemental and molecular species is a key interest in many fields of science. For instance, accurate and quantitative determination of elemental and molecular species is vital for applications in environmental sciences, as well as for material sciences and life sciences.
A fundamental problem for accurate and precise quantitative mass spectroscopy of molecular and elemental species is the interference of other species contained within the same sample. For example, polyatomic (or molecular) ions in a sample may have the same nominal mass as the atomic (or elemental) isotopes to be analyzed, resulting in a mass interference. In other example, different molecular isotopes may have the same nominal mass. As a result, the contribution of each isotopologue to the measured mass spectrum of the sample may be difficult to resolve.
Although two species may have the same nominal mass, due to the systematic nuclear mass defect, ions will have a true mass that is slightly adjusted from their nominal mass. For example, the mass defect causes polyatomic molecules comprising atoms having a nominal nuclear mass less than iron to appear heavier than elemental species having the same nominal atomic mass. Alternatively, for those polyatomic ions comprised of atoms having a nominal mass greater than iron, the polyatomic ions appear lighter than the elemental species at that atomic mass. Therefore, by obtaining a high enough resolution, the mass peak of each isotope can be resolved.
As described in Weyer et al., International Journal of Mass Spectroscopy, 226, (2003), p. 355-368, a double focusing multiple collector inductively coupled plasma (MC-ICP) mass spectrometer can be used to determine isotopic fractions of atomic and polyatomic ions. The detector chamber of the mass spectrometer is equipped with a plurality of Faraday collectors. The Faraday collectors are precisely aligned with respect to atomic and polyatomic ions of the same nominal mass that have been separated at a mass analyzer according to their mass-to-charge ratio. An example of the set-up of one such Faraday collector used in the prior art for measurement of 56Fe and 40Ar16O is shown in FIG. 1. The Faraday collector 116 comprises an aperture or entrance slit 114 arranged in the pathway of the elemental ions 110 and the molecular ions 112 (the elemental and molecular ions having the same nominal mass and the true mass being separated from each other by virtue of the mass defect). The Faraday collector 116 is precisely positioned with respect to the ion beams such that only the elemental ion species 110 enters the entrance slit 114 of the Faraday collector 116. In contrast, the interfering molecular ions 112 are misaligned with the entrance slit 114 and so prevented from entry to the Faraday collector 116.
By adjusting the parameters at the mass analyzer of the mass spectrometer, the atomic ions and polyatomic ions may be “scanned” across the entrance slit of the Faraday collector. The signal received by the Faraday collector during the change in the deflection of the ion beams results in a mass spectrum or mass scan. The mass spectrum represents the intensity or ion current received into the Faraday collector over the extent of the deflection. As an example, a mass spectrum showing Fe isotopes and their respective molecular interferences is shown at FIG. 2A, with FIG. 2B showing a magnified view of a portion of the same mass scan. The resulting mass spectrum demonstrates a number of sloped and plateau regions. A first sloped region occurs when the atomic ion beam moves into the entrance slit, causing the ion current to increase (region A of FIG. 2A). A plateau region occurs where the full atomic ion beam is received through the entrance slit, and so a maximum intensity of the atomic ion species is recorded (region B of FIG. 2A). A second plateau is observed where both the full atomic and polyatomic ion beams are received in the Faraday collector (region C of FIG. 2A). Finally, a third plateau indicates a region where only the polyatomic ion beam is received through the entrance slit at the Faraday collector (region D of FIG. 2A).
In order to obtain a determination of the isotopic ratios present in the sample, Weyer et al. compare the isotopic ratios at different points on the first plateau of the mass spectrum. The diamonds shown in FIG. 2B are 56Fe/54Fe isotopic ratios (see right side y-axis) measured at the marked positions on the plateau. It can be seen that the three data points at the center part of the plateau are in good agreement with each other (within error). However, those data points at the edges of the plateau appear to provide an anomalous result for the isotopic ratio.
Accordingly, the known method of determining isotopic ratios described in Weyer et al. requires broad and flat plateaus in the mass spectrum to provide accurate and consistent measurements. The provision of suitable plateau in samples having ions demonstrating mass interferences relies on a very high resolution separation of the ion species. Furthermore, where temperature fluctuations and mechanical or electrical instabilities occur at the mass spectrometer, these effects can cause drift in the peak position over the measurement time, resulting in inaccuracies in the estimated values.
The mass resolving power can be improved in sector field mass spectrometers by reducing the width of the entrance slit of the Faraday collector. However, this also reduces that overall transmission of ions through the slit, and thereby reduces sensitivity. Increased mass resolving power comes at the cost of ion beam transmission and therefore cannot be increased without limits.
Further limitations of the peak plateau technique described above are apparent in the presence of three mass-interfering ion species. In this circumstance, the ion species with the middle atomic mass does not demonstrate a clean plateau at which a representative signal, independent of the high and low mass ion species, can be identified. Further improving mass resolution by selecting smaller source slits is not feasible because of the significant loss of transmission and ion beam intensity at the detector.
As such, there is required an improved technique for providing high precision quantitative measurements of elemental and molecular species using high resolution mass spectroscopy.